Double digit minor version numbers are very common. Linux was 2.6.34 or something, GNOME was up to 2.32, current Blender is 2.65 etc etc.
KDE major version numbers seem to follow Qt major version numbers, KDE3 was based on Qt3, KDE4 on Qt4, so I'd assume that KDE5 will eventually be based on Qt5.
KDE5 is going to require a significant refactoring of the code base into qt quick 2 and c++11 anyway. Just look at all the things they need to add to kwin (though it is probably the worst case): http://community.kde.org/KWin/Qt5
My understanding is that KDE4 was itself the result of a significant refactoring. My guess is that it's only a matter of time before the conversion happens (and by the looks of it, the devs are already looking into it).
Any KDE based application could in theory start using qtquick instead of qt C++. I don't know why you would take a C++ implementation and make it run on qtscript, though.
I easily see new applications written against kde5 using a lot of qtquick and js implementations. Especially with Ubuntu phone coming, there would be little to change between some gui designs between supporting the desktop and phone.
KDE major version numbers seem to follow Qt major version numbers, KDE3 was based on Qt3, KDE4 on Qt4, so I'd assume that KDE5 will eventually be based on Qt5.